The present invention relates generally to the field of die casting of metal matrix composite materials. More particularly, the present invention relates to die casting of such materials from a semi-solid billet where a solid oxide coating supports a softened interior.
Die casting is a well-known process for forming metal parts by injecting a molten metal into a die cavity, allowing the metal to solidify and removing the part from the die. In using that process, the metal is heated in a crucible to a temperature above its melting point. The liquid metal is poured into an injecting sleeve forming a "shot." A plunger is pushed into the sleeve to slowly displace the material to the entrance of a mold cavity. The plunger then is driven rapidly into the sleeve forcing the shot into the mold cavity. The walls of the mold cavity are cooled so that the metal begins to solidify as it enters the cavity.
Conventional die casting techniques suffer from several problems. Porosity of die cast parts reduces their strength. The range of materials that are readily die cast have relatively low strength and wear resistance. The transfer of liquid metals is cumbersome, may result in contamination of the material from contact with containers, and is a safety hazard to workers and equipment.
Porosity is caused by air and other gases mixed with the metal shot. Gases may become mixed with the metal due to decomposition of impurities, by air entrained in the liquid metal as it is poured into the sleeve or by the injection of air trapped in the sleeve along with the metal shot.
Porosity may be reduced by applying high pressure to the plunger at the end of the injection cycle before the material has solidified. This technique, called compaction, is most effective where solidification is slow. Solidification may be slowed by using a higher melt temperature. However, a higher melt temperature increases shrinkage of the part as it solidifies, reducing the dimensional accuracy of the finished part.
Compaction may also be made more effective by providing an entrance to the mold cavity, called the gate, with a wider cross-section. Gate sizes are limited in conventional die casting processes because the linear velocity of the material entering the mold must remain above a certain minimum, usually about 100 feet per second, to distribute porosity uniformly. Rapid solidification across the restricted gate cross section limits the effectiveness of compaction.
The types of materials that may be used to form the die cast part are limited by the need to inject in a liquid state. Castable materials must have a low enough melting temperature that the die is not damaged by contact with the liquefied materials. As a consequence, die casting is usually limited to alloys of aluminum, magnesium or zinc. Such materials tend to have lower strength and are more prone to wear than higher melting point metals.
A technique known as thixocasting has been proposed to avoid some problems associated with the use of liquid metals in conventional die casting processes. This process is described, for example, in "Rheocasting", Flemings, Merton et al., McGraw-Hill Yearbook of Science and Technology, 1978 , pp. 49-58. A billet of material is formed by first heating a metal alloy to form a liquid melt. The liquid metal then is cooled while it is vigorously agitated. As the metal cools, higher melting point alloys solidify from the liquid in the form of dendritic crystals. Agitation breaks up dendritic crystals into globular particles forming a slurry. The slurry is poured into a preform mold and cast as a billet.
The resulting billet, called a rheocast billet, has a microstructure with a lower melting point continuous phase surrounding a finely dispersed higher melting discrete phase. The rheocast billet then is reheated to a temperature where the continuous phase melts but the discrete phase remains solid, causing the billet to become semi-solid. The semi-solid billet is transferred to an injection molding sleeve and is injected into a cooled die.
The finely dispersed discrete phase gives thixocast materials improved strength over conventionally cast material. Porosity is improved over conventional die casting because liquid metal is not poured into the injection sleeve. Also, handling of semi-solid billets is safer and less cumbersome than transferring liquid metal.
Thixocasting has certain drawbacks. In effective agitation of the rheocast material, especially along the sides of the melting crucible, can allow the formation of large dendritic crystals that may interrupt the flow of material into the die and may mar the surface of the finished part. Porosity of the finished part still is a problem since air and gases may be introduced into the rheocast billet by decomposition of contaminants. Air may also be entrained into the slurry when it is transferred to the preform mold. Controlling the temperature of the semi-solid billet is critical. If the temperature is too high the billet will deform or liquefy before it is transferred to the injection apparatus. If the temperature is too low the material will not flow properly into the mold cavity.
The strength and hardness of low melting point alloys can also be improved by adding ceramic particles to form a metal matrix composite. The composite is less dense than metals with an equivalent tensile strength and can be used to fabricate lighter parts. For example, U.S. Pat. No. 5,486,223 (Carden) describes a method of forming an aluminum-based metal matrix composite wherein boron carbide particles and an aluminum alloy powder are blended, compressed and sintered to form a solid. The resulting material is harder, stronger, stiffer and less dense than castable grades of aluminum alloy alone.
Die cast composite materials made using conventional techniques suffer from the same problems of porosity and cumbersome materials handling as do castings made with metal alloys alone. Additionally, many ceramic materials are not easily wet by certain metal matrix materials. Gases that remain trapped on the ceramic particle surfaces may lead to porosity. High-shear mixing is often required to get these materials to mix. The mixing process may also introduce gases into the material.
Methods of casting metal ceramic composites under vacuum to reduce porosity have been proposed. For example, U.S. Pat. No. 5,322,109 (Cornie) describes a casting method wherein a ceramic material in the form of a fibrous preform is placed inside a mold cavity. Connected to the mold cavity is an infiltrant chamber holding a metal charge. The mold and the infiltrant chamber are heated under a vacuum to melt the charge. The mold and chamber are transferred to a pressure vessel and the liquefied metal charge is forced into the mold cavity, infiltrating the preform. This method can produce parts with desirable strength and hardness properties, and with reduced porosity.
The process is cumbersome, however, requiring the production of preforms for each injected part and the transfer of the molding apparatus from a vacuum furnace to a pressure vessel.